1. Field of the Invention
The invention relates to a brake control apparatus that controls the braking force applied to wheels provided on a vehicle, and a control method thereof.
2. Description of the Related Art
There has been a known brake hydraulic pressure control apparatus equipped with a separation valve that separates a hydraulic pressure circuit into a first system and a second system, and a pressure-intensifying linear control valve provided between the separation valve and a master cylinder in the first or second system (e.g., Japanese Patent Application Publication No. JP-A-Hei-11-115740). In this brake hydraulic pressure control apparatus, when the brake pedal is depressed after the ignition switch has been turned on, a differential pressure corresponding to the relief pressure of the pressure-intensifying linear control valve occurs across the separation valve. On the basis of a change in the differential pressure, leakage of the separation valve is detected. According to this brake hydraulic pressure control apparatus, the leakage failure of the separation valve can be accurately detected.
In vehicles equipped with electric motors as running drive source, such as hybrid vehicles, electric vehicles, etc., a so-called regeneration cooperative control is sometimes performed in which during braking, a requested braking force is generated by using the braking force based on the regeneration and the braking force based on hydraulic pressure in combination. Through the regeneration cooperative control, a part of the kinetic energy occurring during the running of the vehicle is recovered as electric energy during braking. Therefore, the regeneration cooperative control is a factor for improving the fuel economy of a vehicle. In order to further improve fuel economy, it is desirable to begin the regeneration cooperative control promptly after the running drive source of the vehicle is started up.
In the regeneration cooperative control, the hydraulic pressure transferred to the wheel cylinder of each wheel by a brake control apparatus is not a hydraulic pressure pressurized in accordance with the amount of operation of a brake operating member, but is a hydraulic pressure adjusted taking into consideration the braking force provided by the regeneration. If an abnormality is detected on the mechanism that adjusts the hydraulic pressure, the regeneration cooperative control stops, and the hydraulic pressure pressurized by a manual hydraulic pressure source, such as a master cylinder or the like, in accordance with the amount of brake operation is transferred to the wheel cylinder of each wheel, without being adjusted. In such a case, the hydraulic pressure transfer circuit from the manual hydraulic pressure source to each wheel cylinder is constructed so as to be separable into two systems by a separation valve. Therefore, if another failure occurs, for example, if piping in one of the two systems is leaking, the operational system is separated from the failed system by the separation valve, and the operational system may be used to generate braking force. Thus, a construction capable of generating braking force even if two failures concurrently exist is preferable in the light of fail-safe design.
In order to further promote safety, it is desirable that the apparatus be capable of detecting whether or not the separation valve can normally separate the systems, that is, the apparatus be capable of detecting whether or not the separation valve itself has a leak. Although the above-described brake hydraulic pressure control apparatus is capable of detecting any leaks in the separation valve, this capability of the apparatus is based on the precondition that a hydraulic pressure circuit in which a differential pressure is generated across the separation valve is adopted.
However, there are cases where adoption of a different hydraulic pressure circuit construction is desirable in order to improve the controllability of the wheel cylinders, the operability for drivers, etc. Furthermore, as stated above, to improve the fuel economy, it is desirable to begin the regeneration cooperative control promptly after the running drive source of the vehicle starts up. However, in the light of fail-safe design, it is desirable to verify the absence of leaks in the separation valve before beginning the regeneration cooperative control.